1. Field of the Invention
The present invention relates to a method of and apparatus for detecting an end point of plasma treatment, and, more particularly, to a method of and apparatus for detecting an end point of plasma treatment which is suitable for detection of the end point of reaction in the plasma treatment of a specimen such as the substrate of a semiconductor device by selecting a plasma spectrum having a characteristic wavelength from the plasma spectrum generated at the time of reaction in the plasma treatment of such a specimen and by determining said end point from a variation in the quantity of the selected plasma spectrum vis-a-vis the reaction time of the plasma treatment.
2. Description of the Prior Art
A known technique for detecting the end point of reaction in the plasma treatment of a specimen such as a semiconductor device substrate includes selecting plasma spectrum having a characteristic wavelength from the plasma spectrum generated at the time of reaction and detecting the same from a variation in the quantity or intensity of the selected plasma spectrum vis-a-vis the reaction time of the plasma treatment, according to the example which is disclosed in Japanese Patent Laid-Open No. 115536/1981 or Japanese Patent Laid-Open No. 94423/1984.
The radiation corresponding to a reaction product, out of the plasma spectrum generated at the time of reaction in the plasma treatment of a specimen, such as the substrate of a semiconductor device, increases sharply upon starting of the reaction of plasma treatment, reaches its normal state within a short period of time, decreases sharply upon ending of the reaction of plasma treatment, and then assumes its normal state.
According to the technique disclosed in Japanese Patent Laid-Open No. 94423/1984, the end point of reaction in the plasma treatment is determined at the stage when the quantity of the radiation corresponding to a reaction product, i.e., the quantity of plasma spectrum, has decreased sharply upon ending of the reaction of plasma treatment and has assumed its normal state, namely, at the stage when a secondary differential value of a function of the quantity of plasma spectrum/reaction time has reached a preset level. Or, according to the technique disclosed in Japanese Patent Laid-Open No. 115536/1981, the end point of reaction in the plasma treatment is determined at the stage when a primary differential value of the above function of the quantity of plasma spectrum based on the reaction time has reached a preset level.
In these conventional techniques, the primary and the secondary differential values by computation have a time lag from the actual primary and secondary differential values for the following reasons:
(1) sampling of data is conducted at specified intervals, and therefore it is impracticable to increase a data sampling speed infinitely;
(2) a filter is provided in the electric circuit in order to prevent errors in the detection as a result of noise, and the average operation performed with respect to numerical operation is subjected to a time lag due to the effect of such a filter; and
(3) computation of differentiation itself is made by computing the difference between the sampled past data and the sampled present data, from which always only the past differential values can be known.
Meanwhile, in order to prevent erroneous detection of the end point of reaction in the plasma treatment, a period of time from the point of start of the reaction of plasma treatment, for which the detection of the end point of reaction in the plasma treatment is not effected, hereinafter referred to as the disregarding time for plasma treatment end point detection, is established. Namely, the detection of the end point of reaction in the plasma treatment is executed after this disregarding time for treatment end point detection has passed off. This disregarding time corresponds to a range in which the quantity of plasma spectrum increases sharply after the reaction of the plasma treatment starts, and a determination is not made by comparison of the differential values corresponding to the disregarding time with a predetermined reference value for determination.
In the case, for example, where a specimen, such as the substrate of a semiconductor device, is SiO.sub.2 or Al, where the reaction time of the plasma treatment is comparatively long, the end point of reaction in the plasma treatment can be detected by conventional techniques described above. But, in case, for example, such a specimen is Poly-Si, where the reaction time of the plasma treatment is short (the time of plasma etching of Poly-Si is, say, approximately one-tenth of that of plasma etching of SiO.sub.2 of Al), duration of a time of the normal state following the disregarding time for treatment end point detection is short. In such a case, the end point of reaction in the plasma treatment may erroneously be detected by the subjected time lag due to the above reasons (1), (2) and (3), hence there may be a difficulty in accurately detecting the end point of reaction in the plasma treatment.